1. Field of the Invention
The present invention generally relates to the field of low dielectric constant ceramic structures, and their manufacture and, more particularly to ceramic substrates for mounting semiconductor devices, and fluid processing using porous laminated structures. In both applications, the creation of a porous ceramic body with solid, sealed edges and/or external surfaces hold significant advantages.
2. Description of the Prior Art
Future substrate technology requires a dielectric medium with dielectric constant (K) less than 3.0. In general, a low dielectric ceramic substrate can be achieved by using low dielectric materials and well dispersed porosity (air) or a porous (porosity=10 to 50%) body with low K materials. When there is an open pore structure in the substrate, processing fluids in lap and polish and in thin film metallization can enter into the substrate and will cause corrosion and mechanical damage to the substrates. Therefore, it is extremely important that the top, bottom and edges around the substrate are sealed.
Glass ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual glass ceramic layers formed prior to lamination, which upon sintering, will become a sintered dense metal interconnection of metal based conductor.
In the case of ceramic packaging for semiconductor devices, the dielectric constant of the ceramic material is crucial to the performance of the semiconductor devices. A high dielectric constant in the substrate can contribute significantly, particularly in high performance or high frequency application, to both signal propagation delays and noise. Alumina is widely used for moderate performance applications, and provides excellent thermal and mechanical properties. For higher performance applications, glass ceramics and/or glass additions to an alumina matrix are currently used, these materials offer dielectric constants of 5 or less compared to alumina. However, as the density and speed of semiconductor devices increase, still further reductions in dielectric constant will be necessary in order for the ceramic packaging to keep pace with the devices.
The industry target is for a material with dielectric constant of 3 or less. In general, a low dielectric ceramic substrate structure can be achieved by using low dielectric materials and well dispersed porosity (air). A porous body of up to 50% porosity, inherently poses problems in subsequent process operations like, lapping, polishing and thin film metallization. Process fluids trapped in the structure can cause mechanical and structural damage to the substrate. Therefore, it is extremely important that the top, bottom and edges around the substrate are sealed. Another concern of an unsealed porous structure is the surface roughness, which is not an acceptable feature for thin film metallization.
While some polymeric materials can readily achieve this low dielectric constant, they suffer from poor thermal stability and poor thermal expansivity matching with the silicon devices mounted on the substrate. The latter mismatch can contribute to fatigue and reliability concerns at the interconnection between the devices and the substrate. Thin polymeric based film structures have historically been more expensive than ceramic packaging options as well. Ceramic can incorporate large amounts of silicate glass, having dielectric constants of 3.5 or porosity, with air having a dielectric constant of 1. However, both these routes degrade the mechanical performance of the composite ceramic material; high levels of porosity introduce the additional concerns of surface roughness and susceptibility to fluid entrapment.